In 8 inch cast iron skillet or oven-safe frying pan, warm 1 Tbsp grapeseed oil over medium-high heat. Add leek and green onions and cook, stirring often, until softened and lightly browned, about 4 minutes. Add parsley, dill, cilantro, and mixed greens, and continue cooking, stirring constantly, until wilted, about 2 minutes. Tip herb mixture into fine-mesh colander placed in kitchen sink and let any leached moisture drain away. With spoon, press mixture down to ensure herb mixture is not too wet. Set aside. Wipe out skillet and set aside.